Attenuation of Length Dependence of Calcium Activation in Myofilaments of Transgenic Mouse Hearts Expressing Slow Skeletal Troponin I

J Physiol. 2000 Aug 1;526 Pt 3(Pt 3):541-9. doi: 10.1111/j.1469-7793.2000.t01-1-00541.x.


We compared sarcomere length (SL) dependence of the Ca2+-force relation of detergent-extracted bundles of fibres dissected from the left ventricle of wild-type (WT) and transgenic mouse hearts expressing slow skeletal troponin I (ssTnI-TG). Fibre bundles from the hearts of the ssTnI-TG demonstrated a complete replacement of the cardiac troponin I (cTnI) by ssTnI. Compared to WT controls, ssTnI-TG fibre bundles were more sensitive to Ca2+ at both short SL (1.9 +/- 0.1 micrometer) and long SL (2.3 +/- 0.1 micrometer). However, compared to WT controls, the increase in Ca2+ sensitivity (change in half-maximally activating free Ca2+; DeltaEC50) associated with the increase in SL was significantly blunted in the ssTnI-TG myofilaments. Agents that sensitize the myofilaments to Ca2+ by promoting the actin-myosin reaction (EMD 57033 and CGP-48506) significantly reduced the length-dependent DeltaEC50 for Ca2+ activation, when SL in WT myofilaments was increased from 1.9 to 2.3 micrometer. Exposure of myofilaments to calmidazolium (CDZ), which binds to cTnC and increases its affinity for Ca2+, sensitized force developed by WT myofilaments to Ca2+ at SL 1.9 micrometer and desensitized the WT myofilaments at SL 2.3 micrometer. There were no significant effects of CDZ on ssTnI-TG myofilaments at either SL. Our results indicate that length-dependent Ca2+ activation is modified by specific changes in thin filament proteins and by agents that promote the actin-myosin interaction. Thus, these in vitro results provide a basis for using these models to test the relative significance of the length dependence of activation in situ.

Publication types

  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Actin Cytoskeleton / drug effects
  • Actin Cytoskeleton / genetics
  • Actin Cytoskeleton / metabolism*
  • Actins / drug effects
  • Actins / metabolism
  • Animals
  • Azocines / pharmacology
  • Calcium / metabolism*
  • Calcium / pharmacology
  • Cardiotonic Agents / pharmacology
  • Dose-Response Relationship, Drug
  • Enzyme Inhibitors / pharmacology
  • Imidazoles / pharmacology
  • In Vitro Techniques
  • Mice
  • Mice, Transgenic
  • Muscle Contraction / drug effects
  • Myocardium / metabolism*
  • Phosphodiesterase Inhibitors / pharmacology
  • Protein Isoforms / biosynthesis
  • Protein Isoforms / genetics
  • Quinolines / pharmacology
  • Sarcomeres / drug effects
  • Sarcomeres / metabolism
  • Thiadiazines / pharmacology
  • Troponin I / biosynthesis*
  • Troponin I / genetics


  • Actins
  • Azocines
  • Cardiotonic Agents
  • Enzyme Inhibitors
  • Imidazoles
  • Phosphodiesterase Inhibitors
  • Protein Isoforms
  • Quinolines
  • Thiadiazines
  • Troponin I
  • EMD 53998
  • BA 41899
  • calmidazolium
  • Calcium